CONFIG_CPU_MIPS64_R6 is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

MIPS64 Release 6

configname: CONFIG_CPU_MIPS64_R6

CPU type
└─>MIPS64 Release 6
In linux kernel since version 3.10 (release Date: 2013-06-30)  
Choose this option to build a kernel for release 6 or later of the
MIPS64 architecture. New MIPS processors, starting with the Warrior
family, are based on a MIPS64r6 processor. If you own an older
processor, you probably need to select MIPS64r1 or MIPS64r2 instead.